1. Field of the Invention
This invention relates to novel compositions comprising poly(vinyl aromatics) and their use as fluid loss additives in cement slurries having a high salt concentration.
2. Technical Review
The concept of pumping a cement slurry down a wellbore and back up the annulus between the casing and the wellbore in order to set the casing in cement is a well known and well documented commercial practice. Various additives are normally added to the cement slurries used in such instances to achieve specific results or properties in the cement slurry while it is being pumped or in the cement after it is emplaced. For example, dispersants, retarders, fluid loss additives, and the like are normally added to cement slurries prior to pumping and placement of the slurry in the wellbore.
Fluid loss is a severe problem for most cements used in this application and the problem is unusually severe with cement slurries having a high salt concentration. Additives which are known to impart fluid loss control to the slurries tend to be very powerful retarders that prevent the slurry from developing adequate strength for extended periods. Even salt itself retards the setting of the cement when the salt is present in high concentrations.
Other conventional additives also have diverging properties on the cement slurry which make them unsatisfactory. This is illustrated, at least in part, by the three classes of sulfonated poly(vinyl aromatics) cited below which have been used singly, but not in combination, as cement additives.
The sodium salt of sulfonated polynaphthalenes are known dispersants for cement slurries. However they have not been employed in cement slurries as fluid loss additives. In contrast, the sulfonated polystyrenes are known cement dispersants which exhibit good fluid loss control (even at relatively high salt concentrations), but they retard the setting of the cement dramatically and thus, are not suitable for uses as cement dispersants. Also, the sodium salts of sulfonated polyvinyltoluene are unsatisfactory because they are known to be poor dispersants and they can cause gelation of the cement slurry. Sulfonated polyvinyltoluenes have historically been used as fluid loss additives, but are inadequate when used in cement slurries having a high salt content (e.g., from about 17 to about 18 weight percent sodium chloride in the aqueous phase).
Thus, it is generally recognized in the well service industry that the fluid loss additives presently available today are generally inadequate, especially in cement slurries having a high salt concentration.
To the best knowledge of the present inventor, no one has ever taught or suggested that a combination of the above three classes of sulfonated poly(vinyl aromatics) would produce a fluid loss additive that works extremely well, even in the presence of high salt concentration.